Cell-based assays are commonly used for drug discovery to screen large numbers of test chemicals for potential therapeutic activity. Typically, the cells contain a target, such as a protein. Test-chemicals, such as candidate ligands for a target protein, are screened for modulating activity of a target. Screening relies on a detectable change in a property of a cell that faithfully reports target activity in the presence of a test chemical. Many assays use optical methods to detect such activities. Fluorescence detection methods are particularly powerful tools in this regard, because fluorescence detection methods can be sensitive. Many different types of fluorescent probes are available for such assays, including fluorescent probes that act as enzyme substrates, labels for proteins and nucleic acids, indicators of intra-cellular ions, and sensors of membrane voltage.
Despite the recent plethora in available fluorescent tools for assays, fluorescence based assays can be plagued by undesirable, and sometimes intolerable, levels of background fluorescence. For example, solution fluorescence may increase the background fluorescence of the assay sample. Solution fluorescence can obscure a desired signal associated with a fluorescent probe. Solution fluorescence can arise from many sources, including fluorescent probe degradation, targets, cells, various solution components, and test chemicals.
In cell-based assays recently developed by one of the inventors of the present invention, solution fluorescence can give rise to lower signal to noise ratios. These cell-based assays utilize a membrane permeable substrate specific for beta-lactamase, a bacterial enzyme that is not normally expressed in mammalian cells. The substrate diffuses into the cell and is trapped inside the cell by the action of intracellular esterases.
If a cell expresses a beta-lactamase reporter gene, the expressed enzyme will cleave the substrate. Before cleavage the substrate fluoresces green and after cleavage the substrate fluoresces blue. When such assays are used for high-throughput screening, increasing the signal to noise ratio can be advantageous because it increases the sensitivity of the screening system and reliability of the data. Solution fluorescence, however, often thwarts achieving advantageous signal to noise ratios. Solution fluorescence from test chemicals, substrate in the solution, and other solution components that bath the cells contribute to background fluorescence.
The present inventors recognized that membrane compartment assays, such as cell-based assays, that use optical methods could be improved by reducing unwanted light emitted from the solution bathing the membrane compartments, particularly solution fluorescence. The present inventors investigated different washing and incubation methods in an attempt to increase dye loading and retention while reducing solution fluorescence. Although the inventors could reduce solution fluorescence, such manipulations were cumbersome and time consuming.
Consequently, the present inventors developed compositions and methods for reducing the emission undesired light from solutions in membrane compartment assays that did not solely rely on washing or incubation methods. Such compositions and methods are much more applicable to high-throughput screening than improvements to washing and incubation methods.